During wireless communication, a wireless transmitter can encode a message as a digital bit stream (e.g., a stream of logical “1”s and “0”s), and then modulate the digital bit stream onto a carrier wave to generate a stream of symbols. This stream of symbols is then transmitted to an intended wireless receiver through the transmission medium (e.g., through the atmosphere). Upon accurately receiving the stream of symbols, the intended wireless receiver demodulates the symbols and provides the originally transmitted digital bit stream to an end user, often via an audio and/or visual display (e.g., LCD screen and/or speaker).
In carrying out such wireless communication, one type of modulation that the wireless devices can use is polar modulation, wherein amplitude and phase components of a waveform are separated for modulation. FIG. 1a shows an example of a conventional polar modulation circuit that includes a baseband processor 102, a controlled oscillator 104 (e.g., a voltage controlled oscillator or a digitally controlled oscillator), and an amplitude modulation unit 106. The controlled oscillator 104 is configured to generate a high frequency phase modulated (PM) carrier signal, to which amplitude modulation is introduced by providing an amplitude modulated control signal AMHF to the amplitude modulation unit 106. As discussed in more detail below, these components work in coordinated fashion to modulate a digital bit stream onto a carrier wave as a stream of symbols, thereby enabling wireless transmission via an antenna 108.
As one of ordinary skill in the art appreciates, symbols are somewhat akin to an alphabet for communicating wireless devices, in that each symbol has a unique waveform that is different from waveforms of other respective symbols. Symbols often have a unique frequency, amplitude, and/or phase offset relative to other symbols, wherein the phase offset if generally measured relative to a carrier wave known to a transmitter and receiver.
To illustrate one example of how symbols can be used to transmit a digital bit stream, FIG. 1b shows a voltage vs. time plot for two symbols consistent with a binary phase shift keying (BPSK) scheme. Relative to a carrier wave with a 0° phase offset, the first symbol (which can be assigned to a logical “1”) is transmitted with a 45° phase offset. The second symbol (which can be assigned to a logical “0”) is transmitted with a 225° phase offset relative to the carrier wave with 0° phase offset. FIG. 1c shows a phase plot of these symbols. Consistent with FIG. 1c, the first symbol is characterized by a phase offset of 45° and the second symbol is characterized by a phase offset of 225°, wherein both symbols have the same magnitude as evidenced by their equal radii as measured from the origin, which may also be referred to as a “zero crossing point.”
FIG. 1d shows an example of how the symbols of FIGS. 1b-1c can be utilized to transmit a digital bit stream (e.g., a digital bit stream of “101100”). As shown, symbols are transmitted during respective symbol periods. For example, the first symbol (e.g., corresponding to a logical “1” value), is transmitted during a first symbol period extending from TS0 to TS1; the second symbol (e.g., corresponding to a logical “0”) is transmitted during a second symbol period extending from TS1 to TS2; and so on.
As can be seen from the bottom waveform of FIG. 1d (DCO FREQ), a large change in VCO/DCO oscillator frequency is required at symbol boundaries where a 180° phase change occurs. For example, at symbol boundary TS1 the polar modulation transmitter attempts to induce an 180° phase shift to change between the first symbol (used just prior to symbol boundary TS1) and the second symbol (used just after symbol boundary TS1). Because this 180° phase shift requires a near infinite oscillator frequency change, which is difficult to achieve with the limited frequency tuning range that a controlled oscillator allows, polar modulation is difficult to implement.
Hence, the present disclose has developed improved techniques for performing polar modulation. Among other advantages, at least some of these techniques make 180° phase shifts between adjacent symbols easier to achieve compared to conventional techniques.